Viewing-angle dependence is a known problem in liquid crystal display (LCD). High quality image can only be obtained within a narrow range of viewing angle along the normal incidence of light. This angular dependence of viewing is caused by the phase retardation of light traveling through the LCD device. In addition to liquid crystal cell, a pair of crossed polarizers is typically used in an LCD device to direct light propagation in order to yield dark and bright states. With a proper design light can be eliminated at normal incidence, but light leakage occurs at oblique incidence. Considering only the crossed polarizers, the first polarizer rotates the transmission polarization state of the off-axis light by an angle, while the second polarizer rotates the absorption polarization state by another angle in the opposite direction. This leads to incomplete light extinction by the polarizers in the dark state. Further, the birefringent nature of the liquid crystal (LC) molecules used in the LC cell can also cause phase retardation of the off-axis light, giving rise to light leakage. These deficiencies in the function of the crossed polarizers and the LC cell lead to reduction of contrast ratio and color stability in an LC display.
In order to improve the viewing quality of the display, various wave plates have been developed in the art to compensate the phase retardation caused by the components in an LCD device. These wave plates can be either uniaxial or biaxial optical films. The uniaxial optical film has only one optic axis—commonly being referred to as c-axis or extraordinary axis. If the c-axis lies in the direction normal to the film surface, the wave plate is called C-plate. If the c-axis is parallel to the film surface, it is called A-plate. In a uniaxial wave plate, the refractive index along the c-axis (extraordinary index, ne) is either the highest or the lowest, while all other possible axes perpendicular to the c-axis have the same associated refractive index (ordinary index, no). If ne>no, the C- and A-plates are named positive C- and A-plates respectively, whereas if ne<no, they are negative C- and A-plates. Accordingly, the uniaxial wave plates would satisfy the following relationships:                positive C-plate: nz>nx=ny         negative C-plate: nz<nx=ny         positive A-plate: nx>ny=nz         negative A-plate: nx<ny=nz wherein, nx and ny represent in-plane refractive indices, and nz the thickness refractive index.        
If a wave plate has an axis associated with the highest or the lowest refractive index and the axes perpendicular to it have associated refractive indexes differ from one another, the wave plate is said to be optically biaxial. The biaxial wave plate has two optic axes, and it has the relation of nx≠ny≠nz. Let the in-plane index, nx, be the highest or the lowest value, then there exits four types of biaxial wave plates having refractive index profiles of nx>ny>nz, nx>nz>ny, nx<ny<nz, and nx<nz<ny respectively. A wave plate satisfying the equation of nx>ny>nz would have positive in-plane retardation value (Rin) and negative thickness-direction retardation value (Rth), whereas the one satisfying nx<ny<nz would have negative Rin and positive Rth.
Negative C-plate is typically used to compensate TN-LCD (LCD having twisted nematic mode of LC cell) and VA-LCD (LCD having vertically aligned mode of LC cell). In the dark state, the rod-like LC molecules in these two modes are aligned in a homeotropic fashion (normal to the film surface). As a result, the LC cell functions as a positive C-plate in the dark state and thus require a negative C-plate to compensate for phase retardation. As for the positive C-plate, it is often used in combination with positive A-plate to compensate IPS-LCD (LCD having in-plane switching mode of LC cell) for phase retardation arising from the crossed polarizers, the protective TAC (triacetylcellulose) films, and the LC cell.
Negative A-plate may be used in combination with positive A-plate to compensate the angle-dependent characteristic of the crossed polarizers as disclosed in US Patent Application No. 2006/0292372. It can also be used in combination with various A and C plates for the total compensation of the display. Negative A-plate is particularly useful for the compensation of IPS-LCD since the LC cell of which may have a refractive index profile of nx>ny=nz, which is essentially a positive A-plate.
Biaxial wave plates are of interest because they are capable of compensating both in-plane retardation and thickness-direction retardation. Biaxial optical films with various refractive index profiles may be designed to meet the need for specific Rin and Rth compensations in an LCD device.
Among the various types of the compensation plates described above, negative C- and positive A-plates are better known in the art. Polymer films based on polyimide are commonly used for negative C-plate, while stretched films based on polycarbonate or norbornene resin are being used for positive A-plate.
On the contrary, positive C- and negative A-plates are lesser known due to their difficulty in fabrication to achieve the desired refractive index profiles. Various solution-cast polymer films suitable for positive C-plate application have been disclosed in our U.S. patent application Ser. Nos. 11/731,142; 11/731,284; 11/731,285; 11/731,366; and 11/731,367 filed Mar. 29, 2007, the entirety of which is incorporated herein by reference.
US Patent Application No. 2006/0292372 discloses compositions of negative A-plate based on surface active polycyclic compounds.
U.S. Pat. No. 5,189,538 disclosed uniaxially stretching of a solution-cast polystyrene film. The polystyrene film cast from a solution was subject to longitudinal uniaxial stretching at a stretch ratio of 100% at 120° C. After stretching, the refractive indices were found to change from (ηTH 1.551, ηMD 1.548, ηTD 1.548) to (ηTH 1.553, ηMD 1.556, ηTD 1.539), wherein ηTH is a refractive index in the thickness direction (nz), ηMD is a refractive index in the stretching (machine) direction (nx), and ηTD is a refractive index in the transverse (width) direction (ny). Thus, the refractive index profile of the polystyrene film was changed from nx=ny<nz to nx>nz>ny after stretching; among them, the refractive index in the stretching direction (nx) was increased and ny decreased.
U.S. Pat. No. 6,184,957 disclosed LCD having optical compensatory sheet with negative uniaxial property and an optic axis parallel to the plane of the sheet, wherein the optical compensatory sheet satisfies the conditions of 20 nm≦(nx−ny)xd≦1000 nm and 0≦(nx−nz)xd≦200 nm. This patent also disclosed that the optical compensatory sheets can be prepared by stretching uniaxially polystyrene polymers including, for example, polystyrene, polystyrene copolymers, and polystyrene derivatives. In one example, a polystyrene graft copolymer film having a thickness of 70 μm was prepared. The film was then uniaxially stretched 1.9 times at 115° C. The stretched film was reported to have a value of (nx−ny)×d=122 nm. This prior art further disclosed that polymers having negative intrinsic birefringence were suitable for such application; however, it made no differentiation among those polymers in terms of optical properties. Nor did it teach a method for the preparation of polymer films with a refractive index profile of nx<ny=nz that exhibit an exceptionally high in-plane birefringence.
U.S. Pat. No. 7,215,839 discloses biaxial film compositions having the refractive profile of nx>nz>ny based on stretched polynorbornene film. A heat-shrinkable polymer is required for stretching with the polynorbornene to align the film in the thickness direction in order to raise the value of nz over ny. U.S. Pat. No. 6,115,095 discloses a compensation layer composed of biaxial birefringence medium having the greatest principal indices of refraction perpendicular to an LCD substrate; no chemical composition of the biaxial medium is given.